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Issue 6.04

The Astrophysics Spectator

March 6, 2009

Last issue I wrote about how degenerate dwarfs cool from the inside out by emitting neutrinos.  They are not unique, however, in being strong neutrino sources; neutron stars also cool by predominately emitting neutrinos.

Neutrino emission is fundamental to the character of a neutron star.  The very act that creates a neutron star—the gravitational core collapse of a massive star—generates massive numbers of neutrinos that drive way the star's outer layers, producing a supernova.  This neutrino emission is so intense that physicists would see it if the supernova occurred within the Milky Way Galaxy.  The only identifiable object seen by neutrino detectors other than the Sun was the supernova SN 1987A, which occurred in the Large Magellanic cloud, which is a small, nearby galaxy.  As the name implies, this supernova was seen in 1987.

Born in a burst of neutrinos, a neutron star continues emitting a hundred thousand years, until its interior is quite cold.  The power it emits as neutrinos far outstrips the power it emits as x-rays from its photosphere.

Even when hot, we usually do not see neutron stars by the heat that escapes their interiors.  Despite having a surface temperature of over 1 million °K, an isolate, non-spinning neutron star would be difficult to see because of its small surface area.  The radius of a neutron star is of order 15 km, compared to the several thousand kilometer radius of the white dwarf, the nearly 100,000 km radius of the brown dwarf, and the 700,000 km radius of the Sun.  That we can see neutron stars at all is a consequence of the radiation they generate as they spin or of the x-rays generated when they pull gas onto themselves from a companion star.  Without these two mechanisms, neutron stars would be largely invisible.

Next Update:  The next issue of the web site should appear on or shortly after March 18.

Jim Brainerd

Degenerate Objects

The Neutrino Cooling of Neutron Stars.  Neutron stars are strong neutrinos emitters.  The power radiated by a neutron star as neutrinos far outstrips the power radiated as x-rays from the photosphere.  Three processes are responsible for generating the neutrinos: the direct Urca process, the modified Urca process, and the neutrino bremsstrahlung process.  The first process is rapid; it  operates at the cores of the most massive neutron stars.  The remaining-two processes, which operate throughout a neutrons star, cool the neutron star more slowly.  The neutrino emission cools a neutron star in only 100,000 years.  (continue)

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